Nowadays there are many different modulation methods for telecommunication systems. In most cases the resultant modulated signals have a variable amplitude. However, in radio communication systems, constant envelope modulated signals are preferable in most cases due to the existing system constraints in power economy and the consequent use of nonlinear power amplifiers. Thus, it can be said that usually frequency modulation is more suitable for telecommunication systems. Unfortunately, the spectrum of a frequency modulated signal is typically relatively wide. A narrow spectrum is needed particularly in microwave radio systems. In order to narrow the spectrum, a channel filter with a precisely prescribed attenuation and phase characteristic may be used, but this is not preferable in radio systems.
Tamed Frequency Modulation (TFM) has a narrow spectrum and good spectral efficiency and therefore it is suitable for systems where a narrow spectrum is needed, like microwave radio systems.
In radio telecommunication systems a signal propagates on a radio channel as an electromagnetic wave-motion from a transmitter to a receiver. In the channel, the signal alters due to other radio signals, noise and other reasons. Therefore, the received signal differs from the transmitted signal. In the receiver, the received signal has to be detected despite the distortion. Different detection methods have been developed for detecting distorted signals. In the case where a transmitted symbol depends on more than one data symbol, the encoder is said to have memory and decoding is in many cases accomplished by applying trellis. One algorithm which utilizes a trellis-diagram is called Viterbi-algorithm. The Viterbi algorithm leads to the optimum decoding in the maximum likelihood sense.
There is a need to find an effective transmission method when a narrow spectrum, good spectral efficiency and a method for detecting distorted signals are needed.
Short Description
The object of the invention is to provide an improved method and receiver. According to an aspect of the present invention, a method for obtaining a soft decision in detection of a TFM signal is specified. The method comprises: sampling a received signal, reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample and computing path metrics for each survivor path, selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, selecting the survivor with the largest metric as the best survivor, determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
The invention also relates to a second method for obtaining a soft decision in detection of a TFM signal. The second method comprises: sampling a received signal, reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, rotating a carrier phase, computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample and computing path metrics for each survivor path, selecting the path with larger metrics as a survivor from paths entering to a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, selecting the survivor with the largest metric as the best survivor, determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
The invention also relates to a third method for obtaining a soft decision in detection of a TFM signal. The third method comprises: sampling a received signal, reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample using two earlier symbols as an original state of the trellis and computing path metrics for each survivor path, selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, selecting the survivor with the largest metric as the best survivor, determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
The invention also relates to a fourth method for obtaining a soft decision in detection of a TFM signal. The fourth method comprises: sampling a received signal, reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, rotating a carrier phase, computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample using two earlier symbols as the original state of the trellis and computing path metrics for each survivor path, selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, selecting the survivor with the largest metric as the best survivor, determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
According to an aspect of the present invention, a receiver for obtaining a soft decision in detection of a TFM signal is provided. The receiver comprises means for sampling a received signal, means for reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, means for reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, means for computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample and computing path metrics for each survivor path, means for selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, means for selecting the survivor with the largest metric as the best survivor, means for determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
The invention also relates to a second receiver for obtaining a soft decision in detection of a TFM signal. The second receiver comprises means for sampling a received signal, means for reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, means for reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, means for rotating a carrier phase, means for computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample and computing path metrics for each survivor path, means for selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, means for selecting the survivor with the largest metric as the best survivor, means for determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
The invention also relates to a third receiver for obtaining a soft decision in detection of a TFM signal. The third receiver comprises sampling a received signal, means for reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, means for reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, means for computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample using two earlier symbols as an original state of the trellis and computing path metrics for each survivor path, means for selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, means for selecting the survivor with the largest metric as the best survivor, means for determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
The invention also relates to a fourth receiver for obtaining a soft decision in detection of a TFM signal. The fourth receiver comprises means for sampling a received signal, means for reverse rotating the phase of a signal sample in the way that the real part of a complex signal indicates the phase of the sample, means for reverse rotating the phase changes of possible transitions determined by the modulation method used so that the rotation is opposite to the phase belonging to the old state of a trellis diagram, means for rotating a carrier phase, means for computing transition metrics for the state transitions in a trellis diagram with the aid of the real part of the signal sample using two earlier symbols as the original state of the trellis and computing path metrics for each survivor path, means for selecting the path with larger metrics as a survivor from paths entering a state node, and the phase of that survivor is set as the phase state of the new correlative state in question, means for selecting the survivor with the largest metric as the best survivor, means for determining the sign of the demodulated bit with the aid of the last bit of the previous state of the best survivor and calculating the weight of the demodulated bit as the difference of the path metrics of the best survivor and a selected path joining it.
Other preferred embodiments of the invention are disclosed in the dependent claims.
By means of the method of the invention, a weighting coefficient which indicates the reliability of a bit decision can be obtained in addition to hard bit decisions from a TFM demodulator based on the Viterbi algorithm without substantially increasing the complexity of the demodulator or the delay. Weighting coefficients can be utilized particularly when a channel code concatenated serially with TFM modulation is used. A channel decoder, which receives soft decisions from the demodulator, can at best improve coding gain by 2 dB compared to a corresponding decoder which utilizes only hard decisions.
The invention is largely based on a TFM/Viterbi demodulator which outputs hard decisions and utilizes RSSD technique (reduced state sequence detection). In the TFM the phase angle of a signal is determined by adding the rotation of the state defined by three successive data bits to the preceding state. In the RSSD algorithm the phase statuses are not added to the trellis but the preceding state is always calculated according to the survivor to be included in each old state. This yields a four-state trellis where the old state related to a symbol instant consists of two first bits that determine the state rotation and the new state consists of the two last bits. The transition from the old state to the new state is thus defined unambiguously when all the three bits that determine the state rotation are known.
A complex sample read in the channel is divided into eight different branches in a filter bank and phase angle rotation is performed in each branch, the rotation being opposite to the phase angle rotation determined by the transition attached to the branch in question. After this, a second phase angle rotation is performed in each branch, the rotation being opposite to the phase to be provided for the old state. The phase angle of the trellis branch to be attached to the symbol transmitted after these rotations should be zero, and thus simply the real part of a complex sample can be used as the transition metrics. After this, the transition metrics are added to old path metrics and the survivors of the new states are selected according to the new path metrics. The final bit decisions are made at an instant following the survivor length on the basis of the path according to the current best survivor.
Soft decisions are obtained using the path metrics calculated above. The weighting coefficient of a bit is calculated by simply subtracting the metrics of a competing path from the metrics of the best survivor. Since the decision on the best survivor is not made until at an instant following the survivor length, the system requires memory for storing the metrics. The path that will enter the new state together with the best survivor is selected as the competing path, i.e. the path that would have given a bit decision with a different sign and which is rejected at the instant the bit decision is made. Furthermore, since in the TFM errors occur as bursts of two bits, the weighting coefficient is checked at the instant following the bit decision. If the competing path that is to enter the same state with the best survivor gives a smaller weighting coefficient and a bit decision with a different sign, the weighting coefficient of the bit decoded at the preceding instant is replaced with a smaller weighting coefficient. However, bit decisions, i.e. the sign of the bit to be decoded, are always determined according to the best survivor.
By means of the invention soft decisions can be obtained from the TFM demodulator, which can be implemented by a relatively simple solution.